The present invention relates to a temperature control device for a liquid crystal optical shutter in a recording apparatus using an optical recording scheme utilizing an electrooptical effect.
Various types of optical recording apparatuses using an optical transducer element such as a laser, an OFT, an LED (Light-Emitting Diode), and an LCD (Liquid Crystal Display) have been proposed. Among these apparatuses, a recording apparatus using a liquid crystal optical shutter has received a great deal of attention since it has many advantages such as high printing quality, high-speed operation, and low noise. A conventional recording apparatus of this type uses a liquid crystal optical shutter as a recording head. The optical shutter is selectively opened/closed on the basis of the recording signal to selectively transmit light from a light source through the optical shutter, thereby writing information on a photosensitive body.
Two-frequency driving is known as a liquid crystal optical shutter driving method utilizing inversion of dielectric anisotropy of liquid crystal molecules upon changes in frequency of the electric field. As shown in FIG. 1, the dielectric anisotropy of the liquid crystal is positive at a frequency lower than zero-crossing frequency f.sub.c. However, when the frequency is higher than frequency f.sub.c, the dielectric anisotropy is inverted to the negative polarity. If frequency f.sub.L lower than frequency f.sub.c is used as a frequency for an electric field, liquid crystal molecules are oriented parallel to the electric field and the optical shutter can be open. However, if frequency f.sub.H higher than frequency f.sub.c is used as a frequency for the electric field, liquid crystal molecules are aligned in a direction perpendicular to the electric field, thereby closing the optical shutter. By controlling the ON/OFF operation of the optical shutter, it can suitably serve as a recording head for the photosensitive body.
The dielectric anisotropy of the liquid crystal is very sensitive to its viscosity and therefore tends to greatly change according to changes in temperature. If the viscosity of a liquid crystal is changed, f.sub.c is changed accordingly. For example, if the temperature is changed from 20.degree. C. to 40.degree. C., zero-crossing frequency f.sub.c is changed from, e.g., 5 kHz to 46 kHz, and the liquid crystal shutter cannot serve as an optical shutter. For this reason, constant temperature control is required for the optical shutter. Furthermore, if a liquid crystal has a low viscosity, behavior of liquid crystal molecules is fast and high-speed operation can be expected. For this reason, the liquid crystal shutter is used at a relatively high temperature.
A typical conventional two-position control type temperature control device for controlling a liquid crystal shutter is illustrated in FIG. 2.
Referring to FIG. 2, reference symbol H denotes a heater arranged in a liquid crystal optical shutter (not shown). Energization of heater H is controlled such that switching of transistor Q.sub.B is controlled in response to an output from comparator Q.sub.A. In this manner, the temperature of the liquid crystal optical shutter is controlled. More specifically, voltage V.sub.N obtained by causing thermistor TH and resistor R.sub.A, both of which are arranged in the liquid crystal optical shutter, to divide power source voltage V.sub.10 is input to noninverting terminal I.sub.N of comparator Q.sub.A to detect a temperature of the liquid crystal optical shutter. At the same time, voltage V.sub.I obtained by causing resistors R.sub.B and R.sub.C to divide voltage V.sub.10 is input to inverting terminal I.sub.I of comparator Q.sub.A. If the temperature of the liquid crystal optical shutter is low, thermistor TH has a high resistance. If V.sub.N &gt;V.sub.I, then an output from comparator Q.sub.A goes high to turn on transistor Q.sub.B, so that heater H is energized to heat the liquid crystal optical shutter. However, if the temperature of the liquid crystal optical shutter becomes high, thermistor TH has a low resistance. If V.sub.N &lt;V.sub.I, then the output from comparator Q.sub.A goes low to deenergize heater H, so that heater H is no longer heated. Such a two-position control type temperature control device is described in Japanese Patent Disclosure (Kokai) No. 52-101058.
In the conventional temperature control device described above, since heater H is energized when the temperature of the liquid crystal optical shutter is lower than the reference temperature and is deenergized when the shutter temperature is higher than the reference temperature, sufficient temperature control precision cannot be obtained, as shown in FIG. 3. In particular, when an ambient temperature is changed, stable operation cannot be performed. For this reason, the operating characteristics of the liquid crystal optical shutter, that is, the opening of the shutter, becomes unstable. A latent potential becomes nonuniform upon writing of information on the photosensitive body, variations in image density occur during development, and image quality is thus degraded.
Japanese Patent Disclosure No. 57-117980 and U.S. Pat. No. 4,386,836 describe printers whose temperature control is performed upon energization of a heater. However, no prior art apparatuses are found to perform fine temperature control even according to changes in ambient temperature.
Thermistor TH and heater H are formed in contact with, e.g., a glass substrate constituting a liquid crystal optical shutter. The temperature detected by thermistor TH upon energization of heater H is lowered by a temperature of the glass substrate on which heater H is formed. It takes a given period of time to equalize the substrate temperature and the temperature detected by thermistor TH. More specifically, it takes a given period of time to cause heater H to heat the glass substrate and to conduct heat to thermistor TH. The given period of time is prolonged when the ambient temperature of the liquid crystal optical shutter is low. For this reason, when thermistor TH detects a suitable temperature, the actual temperature of the glass substrate contacting heater H exceeds the suitable temperature, thus resulting in overshooting, as indicated by F in FIG. 3. As a result, stable temperature control cannot be achieved.